Silicon carbide (SiC) is one type of semiconductor material, and has a larger band gap compared with other semiconductor materials, such as silicon (Si) and gallium arsenide (GaAs). Therefore, investigation for producing silicon carbide devices using a silicon carbide substrate, such as power devices, high frequency devices, devices operating at high temperatures has been conducted, as shown in Japanese Unexamined Patent Application First Publication No. 2003-243323.
The power device having a silicon carbide substrate includes a silicon carbide substrate, a circuit element which has a power device element and is formed on the silicon carbide substrate, and an electrode pad which is formed on the circuit element and electrically connected with the power device.
The electrode pad has a fine shape. Therefore, the electrode pad has been produced by a lift-off method that is able to form a fine metal pattern.
Specifically, the electrode pad is formed on the circuit element by depositing a metal film which becomes a base material of the electrode pad from the upper side of a resist having an opening exposing a surface of the circuit element layer at which the electrode pad will be formed, and then lifting the resist having the metal film off the circuit element layer. When the electrode pad is formed by this method, it is necessary to form the opening such that the sectional form of the opening be an inverse tapered shape. Therefore, it is preferable that the resist used in the lift-off method be a positive-negative reverse type resist.
On the other hand, when the power device is produced using the electrode pad, in order to electronically connect between a wiring substrate and the power device, the electrode pad is connected with a metal wire by a wire bonding method. Due to this fact, the electrode pad has to have a thickness sufficient to have excellent mechanical strength.
In addition, since a high current flows through the electrode pad constituting the power device, the power device is easily heated. Therefore, it is necessary to radiate heat of the power device through the electrode pad. In other words, it is necessary to use the electrode pad as a heat radiator.
Based on these reasons, it is necessary to adjust the thickness of the electrode pad in the silicon carbide device having a silicon carbide plate to 6 μm or more.
However, when a commercial positive-negative reverse type resist is used, the maximum thickness of the film formed at a time is about 6 μm at the present time. Therefore, the thickness of the electrode pad, which can be formed by the lift-off method using a commercial positive-negative reverse type resist, is about 5 μm. Due to this fact, it is impossible to produce the electrode pad having a thickness of 6 μm or more by a conventional production method.
Moreover, it can be thought that the thickness of the positive-negative reverse type resist can be 7 μm or more by laminating twice the positive-negative reverse type resist having the same properties. In this case, it is impossible to adjust the in-plane irregularity (non-uniformity on the surface of the silicon carbide substrate) on the surface of the formed positive-negative reverse type resist to a desired range (for example, when the outer circumference of the silicon carbide substrate having a diameter of 3 inches is cut such that the cutting width be 5 mm (edge cutting at 5 mm), the variation of the thickness of the resist formed on the residual center silicon carbide substrate is 5% or less).
That is, it is impossible to produce the electrode pad which has a thickness of 6 μm or more and an excellent shape on the surface of the silicon carbide substrate. Specifically, it is impossible to produce the electrode pad having a thickness of 6 μm or more and having no burring on the edge.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide a method for producing a thick film metal electrode that is able to form a positive-negative reverse type resist, which has a thickness of 7 μm or more and excellent in-plane uniformity, on the circuit element formed on the silicon carbide substrate, and a method for producing a thick film resist.
In other words, the present invention provides the following solutions.    (1) A method for producing a thick film metal electrode having a thickness of 6 μm or more on an upper surface of a circuit element layer formed on a silicon carbide substrate including:
a step of treating the upper surface of the circuit element layer with hexamethyldisilazane;
a step of forming a first positive-negative reverse type resist having a first viscosity on the upper surface of the circuit element layer which is treated with hexamethyldisilazane;
a step of forming a second positive-negative reverse type resist having a second viscosity, which is larger than the first viscosity, on the first positive-negative reverse type resist such that a total thickness of the first and second positive-negative reverse type resists be 7 μm or more;
a step of exposing the first and second positive-negative reverse type resists through a mask for exposure including a shading part facing an electrode formation area at which the thick film metal electrode is formed;
a step of baking the first and second positive-negative reverse type resists to reverse positive properties of the first and second positive-negative reverse type resists to negative properties, after the exposing;
a step of exposing the whole first and second positive-negative reverse type resists after the baking;
a step of developing the first and second positive-negative reverse type resists to form an opening, which has a sectional form having an inverse tapered shape and exposes the electrode formation area, after the exposing step for the whole first and second positive-negative reverse type resists;
a step of post-baking the first and second positive-negative reverse type resists after forming the opening;
a step of forming a metal film having a thickness of 6 μm or more, which is a base material for the thick film metal electrode by a vapor deposition method from the upper side of the second positive-negative reverse type resist, after the post-baking; and a step of lifting the first and second positive-negative reverse type resists off.    (2) The method for producing a thick film metal electrode according to (1), wherein the first viscosity is 30 cp or less, and the second viscosity is 80 cp or more.    (3) The method for producing a thick film metal electrode according to (1) or (2), wherein the thick film metal electrode is an electrode for external connection to be connected with a metal wire by a wire bonding method.    (4) The method for producing a thick film metal electrode according to any one of (1) to (3), wherein the circuit element layer includes a power device, and the power device is electrically connected with the thick film metal electrode.    (5) The method for producing a thick film metal electrode according to any one of (1) to (4), wherein the step of forming a first positive-negative reverse type resist includes a step of forming the first positive-negative reverse type resist in a liquid state on the upper surface of the circuit element layer, which is treated with hexamethyldisilazane; and a step of pre-baking the first positive-negative reverse type resist in a liquid state to harden the first positive-negative reverse type resist.    (6) The method for producing a thick film metal electrode according to (5), wherein the thickness of the hardened first positive-negative reverse type resist is 5 μm or less in the step of forming a first positive-negative reverse type resist.    (7) The method for producing a thick film metal electrode according to any one of (1) to (6), wherein the step of forming the second positive-negative reverse type resist includes a step of forming the second positive-negative reverse type resist in a liquid state on a hardened first positive-negative reverse type resist; and a step of pre-baking the second positive-negative reverse type resist in a liquid state to harden the second positive-negative reverse type resist.    (8) The method for producing a thick film metal electrode according to (7), wherein the thickness of the hardened second positive-negative reverse type resist is 3 μm or more in the step of forming the second positive-negative reverse type resist.    (9) A method for producing a thick film resist having a thickness of 7 μm or more on an upper surface of a circuit element layer formed on a silicon carbide substrate including:
a step of treating the upper surface of the circuit element layer with hexamethyldisilazane;
a step of forming a first positive-negative reverse type resist having a first viscosity on the upper surface of the circuit element layer which is treated with hexamethyldisilazane; and
a step of forming a second positive-negative reverse type resist having a second viscosity, which is larger than the first viscosity, on the first positive-negative reverse type resist such that a total thickness of the first and second positive-negative reverse type resists be 7 μm or more.    (10) The method for producing a thick film resist according to (9), wherein the first viscosity is 30 cp or less, and the second viscosity is 80 cp or more.    (11) The method for producing a thick film resist according to (9) or (10), wherein the method further includes:
a step of exposing the first and second positive-negative reverse type resists through a mask for exposure including a shading part;
a step of baking the first and second positive-negative reverse type resists to reverse positive properties of the first and second positive-negative reverse type resists to negative properties, after the exposing;
a step of exposing the whole first and second positive-negative reverse type resists after the baking; and
a step of developing the first and second positive-negative reverse type resists, after the exposing step for the whole first and second positive-negative reverse type resists.    (12) The method for producing a thick film resist according to any one of (9) or (11), wherein the step of forming a first positive-negative reverse type resist includes a step of forming the first positive-negative reverse type resist in a liquid state on the upper surface of the circuit element layer, which is treated with hexamethyldisilazane; and a step of pre-baking the first positive-negative reverse type resist in a liquid state to harden the first positive-negative reverse type resist in a liquid state.    (13) The method for producing a thick film resist according to (12), wherein the thickness of the hardened first positive-negative reverse type resist is 5 μm or less in the step of forming a first positive-negative reverse type resist.    (14) The method for producing a thick film resist according to any one of (9) to (13), wherein the step of forming the second positive-negative reverse type resist includes a step of forming the second positive-negative reverse type resist in a liquid state on a hardened first positive-negative reverse type resist; and a step of pre-baking the second positive-negative reverse type resist in a liquid state to harden the second positive-negative reverse type resist in a liquid state.    (15) The method for producing a thick film resist according to (14), wherein the thickness of the hardened second positive-negative reverse type resist is 3 μm or more in the step of forming the second positive-negative reverse type resist.
In the present invention, the first positive-negative reverse type resist having the first viscosity is formed on the upper surface of the circuit element layer which has been treated with hexamethyldisilazane (HMDS). Therefore, it is possible to produce the first positive-negative reverse type resist having a uniform thickness such as 5 μm or less.
In addition, in the present invention, the second positive-negative reverse type resist having the second viscosity, which is larger than the first viscosity, is formed on the first positive-negative reverse type resist. Therefore, it is possible to produce the second positive-negative reverse type resist having a uniform thickness such as 3 μm or more on the first positive-negative reverse type resist.
Thereby, it is possible to adjust the in-plane irregularity (non-uniformity) of the positive-negative reverse type resist, which includes the first and second positive-negative reverse type resists, and has a thickness of 7 μm or more, to a desired range (for example, when the outer circumference of the silicon carbide substrate having a diameter of 3 inches is cut such that the cutting width be 5 mm (edge cutting at 5 mm), the variation of the thickness of the positive-negative reverse type resist formed on the residual center silicon carbide substrate is 5% or less).
In addition, in the present invention, the first and second positive-negative reverse type resists are exposed through a mask for exposure including a shading part facing an electrode formation area at which the thick film metal electrode is formed, the first and second positive-negative reverse type resists are baked to reverse positive properties of the first and second positive-negative reverse type resists to negative properties, the whole first and second positive-negative reverse type resists are exposed, and then the first and second positive-negative reverse type resists are developed. Thereby, it is possible to form an opening, which has a sectional form having an inverse tapered shape and exposes the electrode formation area, in the first and second positive-negative reverse type resists (thick film resist).
Furthermore, in the present invention, after forming the opening, the first and second positive-negative reverse type resists are post-baked, the metal film having a thickness of 6 μm or more, which is a base material for the thick film metal electrode, is formed by a vapor deposition method from the upper side of the second positive-negative reverse type resist, and the first and second positive-negative reverse type resists (thick film resist) are lifted off. Thereby, it is possible to produce the thick film metal electrode which has a thickness of 6 μm or more and has excellent formability (that is, no burring at edges) in the area of the silicon carbide substrate.
The method for producing a thick film metal electrode and the method for producing a thick film resist according to the present invention are useful in producing a thick film metal electrode in a silicon carbide device including a silicon carbide substrate and a thick film resist used in forming the thick film metal electrode by a lift-off method.